The present invention relates to a seal for the interstice between the confronting surfaces of two relatively rotatable machine parts. At least one of the two confronting surfaces is provided with at least one helical groove that imparts a pumping action toward the space to be sealed. The helical groove is preceded on the side remote from the sealed space by a preliminary seal.
A seal of this type is disclosed in the German Pat. Publication (DAS) No. 24 53 118. The preliminary seal in this case consists of screw threads which, on the side facing the ambient air, adjoin the area provided with the helical grooves. The pumping action that results when the shaft rotates is, like that of the helical grooves, in the direction of the sealed space. This pumping action results in a suction of the outside air and draws foreign particles into the area of the dynamic sealing zone. Considerable wear can be caused by this effect.